Photoacoustic matching material and human tissue simulation material

ABSTRACT

Since a medium having light propagation properties and sound propagation properties similar to those of human tissues is provided, reflection of light and an acoustic wave at a surface of a test portion is prevented, and accuracy control of a photoacoustic wave diagnostic apparatus is realized. A photoacoustic matching material is provided which includes a polyol, an inorganic oxide, and a pigment, the latter two being dispersable in the polyol, and in this photoacoustic matching material, a dispersed amount of the inorganic oxide to the polyol is in a range of 0.10 to 0.25 percent by weight, and a dispersed amount of the pigment to the polyol is in a range of 0.0001 to 0.0005 percent by weight.

TECHNICAL FIELD

The present invention relates to a photoacoustic matching material and ahuman tissue simulation material, and more particularly relates to aphotoacoustic matching material and a human tissue simulation material,each of which has optical properties and acoustic properties similar tothose of human tissues.

BACKGROUND ART

A photoacoustic wave diagnostic apparatus is an apparatus in which, whena living body which is a test portion to be tested is irradiated withlight, an image is displayed based on a detection signal of an acousticwave (typically an ultrasonic wave) generated by thermal expansion of ameasuring object. By this diagnostic apparatus, a specific material inthe test portion, such as glucose or hemoglobin contained in blood orthe like, is tested.

In the photoacoustic wave diagnostic apparatus, a probe is used toreceive an acoustic wave. When an air layer is present between the probeand a living body and/or when the acoustic impedance of the probe issignificantly different from that of a living body, an acoustic wave isreflected at the interface therebetween. For this reason, it isnecessary to provide an acoustic matching material having acousticproperties similar to those of human tissues between the probe and aliving body. In PTL 1, an acoustic matching material used for anacoustic wave probe and a manufacturing method thereof have beendisclosed, the acoustic matching material having acoustic propertiessimilar to those of human tissues and being formed from a base materialof an elastomer or a resin and a composite powder having a higheracoustic impedance than that of the base material.

In addition, in a medical diagnostic apparatus, for the purposes ofaccuracy control and training of engineers, a human tissue model calleda human tissue simulation material (hereinafter, referred to as a“phantom” in some cases) has been used. As a material for the phantom, amaterial which has properties similar to those of human tissues andwhich can be stored for a long period of time without causing, forexample, the growth of bacteria has been desired.

In addition, PTL 2 has disclosed a non-corrosive acoustic wave phantomwhich has acoustic properties similar to those of human tissues andwhich is obtained by dispersing a powdered organic filler in a urethaneresin functioning as a base material.

CITATION LIST Patent Literature

-   PTL 1 Japanese Patent Laid-Open No. 2009-71393-   PTL 2 Japanese Patent Laid-Open No. 2003-310610

SUMMARY OF INVENTION

Since the optical properties of the acoustic matching material disclosedin PTL 1 and those of the phantom disclosed in PTL 2 are different fromthe optical properties of human tissues, when the above materials areeach used for a photoacoustic wave diagnostic apparatus, an acousticwave is generated by scattering and/or absorption of light at aninterface with a human body which is a subject to be tested. As aresult, clear imaging in the vicinity of the interface may not be easilyperformed.

Although a material having light scattering properties is also includedin the composite powder disclosed in PTL 1, since the volume fractionthereof is in a range of 50 to 70 percent by weight, the lightscattering properties thereof are significant as compared to the opticalproperties of human tissues, and hence the optical properties of thecomposite powder may not be easily made similar to those of humantissues. In addition, in the case of the phantom disclosed in PTL 2 inwhich the powdered organic filler is used, the acoustic propertiesthereof may also be made similar to those of human tissues. However,since light absorption properties and light scattering properties arehardly obtained by a dispersed amount of the powdered organic fillerdisclosed in PTL 2, the optical properties thereof may not be easilymade similar to those of human tissues.

Hence, the present invention provides a matching material and/or aphantom having not only acoustic properties similar to those of humantissues but also optical properties similar thereto.

In order to solve the above problems, through intensive research carriedout by the present inventors, a photoacoustic matching medium having notonly acoustic properties similar to those of human tissues but alsooptical properties similar thereto was discovered. A photoacousticmatching medium of the present invention includes a polyol, an inorganicoxide, and a pigment, the latter two being dispersable in the polyol,and in the above photoacoustic matching medium, a dispersed amount ofthe inorganic oxide to the polyol is in a range of 0.10 to 0.25 percentby weight, and a dispersed amount of the pigment to the polyol is in arange of 0.0001 to 0.0005 percent by weight.

In addition, the photoacoustic matching medium of the present inventionmay be used as a photoacoustic matching material or a phantom in aphotoacoustic wave diagnostic apparatus.

Since having light propagation properties and sound propagationproperties, both of which are similar to those of human tissues, whenthe photoacoustic matching material and the phantom of the presentinvention are each used in a photoacoustic diagnostic apparatus,reflection of an acoustic wave at an interface of a test portion andgeneration of an acoustic wave by irradiation light are suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a photoacoustic wave diagnostic apparatusin which a photoacoustic matching material of the present inventionwhich has light propagation properties and sound propagation propertiessimilar to those of human tissues is placed at a test portion.

FIG. 2 is a schematic view showing a probe unit in which in the presentinvention, a light irradiation portion and an acoustic wave probe areintegrated with each other.

FIG. 3A is a schematic view showing the case in which a container formedfrom a photoacoustic matching material of the present invention isapplied to a photoacoustic wave diagnostic apparatus.

FIG. 3B is a schematic view showing the case in which a test portion isplaced in the container formed from the photoacoustic matching materialof the present invention.

FIG. 4 is a schematic view showing the case in which a photoacousticmatching material of the present invention is used as a phantom for aphotoacoustic wave diagnostic apparatus.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings and the like. The following embodiments areexamples of a photoacoustic matching material and a human tissuesimulation material (phantom) of the present invention; however, thepresent invention is not limited to the embodiments.

In addition, the importance of the present invention is a discovery of amatching material which is formed from a non-corrosive polyol used as abase material and which has acoustic properties and optical propertiessimilar to those of a human body.

First Embodiment

A photoacoustic matching material according to a first embodiment of thepresent invention includes a polyol, an inorganic oxide, and a pigment,the latter two being dispersable in the polyol, and a dispersed amountof the inorganic oxide to the polyol is in a range of 0.1 to 0.25percent by weight. In addition, a dispersed amount of the pigment to thepolyol is in a range of 0.0001 to 0.0005 percent by weight. When thedispersed amounts are controlled as described above, a photoacousticmatching material having acoustic properties and optical propertiessimilar to those of a human body can be obtained.

As a polyol used for the present invention, although a polyether polyol,a polyester polyol, a polycarbonate polyol, and the like may bementioned, in terms of the similarity to the acoustic properties of ahuman body, a polyether polyol is more preferably used. In terms of thesimilarity to the sound propagation properties of a human body and thestability of a resin, the polyether polyol is preferably a copolymerwhich is formed from ethylene oxide and propylene oxide at a molar ratioin a range of 30:70 to 70:30 and which has a number average molecularweight in a range of approximately 6,000 to 8,000.

A polyol used for the photoacoustic matching material of the presentinvention is generally in a liquid state (including a gel state). Ifnecessary, the polyol may be cured into a solid state by adding a curingagent. Although a curing agent used in this case is not particularlylimited, in the present invention, an isocyanate compound is preferablyused. As the isocyanate compound used in the present invention as acuring agent, a compound which can enable a polyol to have soundpropagation properties similar to those of human tissues is preferable.As a particular example of the isocyanate compound described above, forexample, hexamethylene diisocyanate (HDI), diphenylmethane diisocyanate(MDI), tolylene diisocyanate (TDI), isophorone diisocyanate (IPDI), orxylylene diisocyanate (XDI) may be mentioned.

In addition, when titanium oxide which is an inorganic oxide isdispersed in a polyol as a filler having light scattering properties,since the acoustic attenuation of the resin increases, a resin having alow acoustic attenuation is preferable. Since the acoustic attenuationof a resin prepared using HDI is very low, such as −0.12 dB/cm/MHz, HDIis particularly preferable. Although the case in which HDI is used willbe described in this embodiment, in the present invention, othermaterials may also be used.

When the number average molecular weight of a photoacoustic matchingmaterial of the present invention containing a polyol and an isocyanatecompound is high, the hardness is likely to decrease. In addition, whenthe number average molecular weight is low, the hardness is likely toincrease. In terms of the similarity to the sound propagation propertiesof a human body, the number average molecular weight of thephotoacoustic matching material of the present invention is preferablyin a range of approximately 1,000 to 10,000.

As the filler having light scattering properties as described above, forexample, an inorganic oxide, such as titanium oxide, may be mentioned.However, titanium oxide having no surface modification precipitates in apolyol and is not likely to be dispersed therein. For this reason, inorder to uniformly disperse titanium oxide in a polyol, a coating filmis preferably formed on the surface of titanium oxide. Although the casein which titanium oxide processed by a surface treatment using aluminumoxide and hexamethyldisilazane will be described in this embodiment, inthe present invention, other materials may also be used. In addition,the dispersibility may be improved not only by forming a coating filmbut also by performing various surface modification techniques, such asmethylation and ethylation, on the surface.

In the present invention, in view of light diffusion, the averageparticle diameter of the titanium oxide processed by a coating treatmenton the surface thereof using aluminum oxide is preferably in a range of0.2 to 0.3 μm. In this embodiment, although the case in whichsurface-treated titanium oxide having an average particle diameter of0.21 μm is used will be described, in the present invention, titaniumoxide having a different particle diameter may also be used.

As a filler having light absorption properties used for the presentinvention, a pigment is preferably used. As the pigment, for example,there may be mentioned a black pigment such as carbon black; a cyanpigment such as copper phthalocyanine; a magenta pigment, such as amonoazo lake pigment or a monoazo pigment; and a yellow pigment, such asdiarylide yellow. However, since a pigment itself precipitates in apolyol and is not dispersed therein, a dispersion in which a pigment iscovalently bonded to a polyol is preferably used. In the presentinvention, although a dispersion in which carbon black is covalentlybonded to a polyether polyol is used, other polyols and pigments mayalso be used.

Hereinafter, a method for preparing a medium having light propagationproperties and sound propagation properties similar to those of humantissues according to an embodiment of the present invention will bedescribed. After a filler is dispersed in a polyol received in a beakerand is stirred, vacuum defoaming is performed. When resin curing isperformed, after the vacuum defoaming is performed, an isocyanatecompound functioning as a curing agent is added to the polyol, andsubsequently, a mixture thus obtained is charged in a predetermined moldand is then heated at 90° C. for 1 hour.

Measurement results of the acoustic velocity and the acousticattenuation are shown in Table 1 which are obtained when as a fillerhaving light scattering properties, titanium oxide processed by asurface treatment using aluminum oxide and hexamethyldisilazane isdispersed in a resin including a polyol and an isocyanate compound. Asthe polyol, a copolymer (number average molecular weight: 7,000) ofethylene oxide and propylene oxide at a molar ratio of 50:50 was used.As the isocyanate compound, HDI was used. The addition amount of HDI wasset to 3.4 percent by weight to the polyol. The average particlediameter of the titanium oxide processed by a surface treatment usingaluminum oxide and hexamethyldisilazane was adjusted to 0.21 μm. InTable 1, the dispersed amount indicates a weight ratio of the titaniumoxide to the polyol and is represented by percent by weight (wt %).

TABLE 1 DISPERSED ACOUSTIC ACOUSTIC AMOUNT OF VELOCITY ATTENUATION TiO₂[wt %] [m/s] [dB/cm/MHz] 0.00 1427.3 0.12 0.05 1420.5 0.22 0.10 1415.20.38 0.15 1402.8 0.46 0.20 1393.6 0.57 0.25 1383.5 0.68 0.50 1378.5 0.840.75 1362.4 1.08 1.00 1352.5 1.25

Since the acoustic velocity and the acoustic attenuation in a living fattissue are in a range of 1,350 to 1,450 m/s and in a range of −0.20 to−1.3 dB/cm/MHz, respectively, the acoustic velocity and the acousticattenuation of the photoacoustic matching material are also requiredwithin the above acoustic velocity range and acoustic attenuation range,respectively. The acoustic impedance defined by a product of the densityof the photoacoustic matching material and the acoustic velocity thereinis 1.5 MRayl. Since the density of the resin prepared in this embodimentis 1.13 regardless of the dispersed amount of titanium oxide, acousticproperties similar to those of a living body may be obtained byadjustment of the dispersed amount of titanium oxide. Hence, when thedispersed amount of the inorganic oxide to the polyol is set in a rangeof 0.1 to 1.0 percent by weight, a matching material having acousticproperties similar to those of a living body can be obtained. Inaddition, when the dispersed amount of titanium oxide was more than 1.0percent by weight, a curing defect occurred, and hence a cured resincould not be obtained.

Next, in the case in which resin curing was not performed on a polyol(HDI was not added), and titanium oxide was dispersed in a liquidpolyol, the acoustic velocity and the acoustic attenuation weremeasured, and the results thereof are shown in Table 2. As in the casein which the resin curing was performed, as the polyol, a copolymer(number average molecular weight: 7,000) of ethylene oxide and propyleneoxide at a molar ratio of 50:50 was used. As the filler, titanium oxide(average particle diameter: 0.21 μm) processed by a surface treatmentusing aluminum oxide and hexamethyldisilazane was used. In the table,the dispersed amount indicates a weight ratio of the titanium oxide tothe polyol and is represented by percent by weight.

TABLE 2 DISPERSED ACOUSTIC ACOUSTIC AMOUNT OF VELOCITY ATTENUATION TiO₂[wt %] [m/s] [dB/cm/MHz] 0.00 1475.5 0.09 0.05 1458.2 0.13 0.10 1449.40.26 0.15 1445.5 0.39 0.20 1442.2 0.51 0.25 1438.5 0.58 0.50 1431.2 0.780.75 1426.8 0.85 1.00 1421.2 0.95

From the results shown in Tables 1 and 2, it was found that, when noresin curing was performed, although the acoustic velocity was fasterthan that obtained when resin curing was performed, the acousticproperties were not significantly different from those of human tissues.From the above results, it was found that by adjustment of the particlediameter and the dispersed amount of titanium oxide with respect to thepolyol, acoustic properties similar to those of a living body could beobtained. That is, even when the resin is not cured, if the dispersedamount of the inorganic oxide to the polyol is set in a range of 0.1 to1.0 percent by weight, a matching material having acoustic propertiessimilar to those of the living body (fat tissue) can be obtained.

Since the viscosity of the liquid polyol is approximately 700 mPa·s at25° C. regardless of whether titanium oxide is dispersed or not, thepolyol is in a gel state having liquidity at room temperature. Inaddition, since titanium oxide and the polyol are harmless to a humanbody, the polyol containing titanium oxide dispersed therein can be usedas an acoustic matching material which is applied to a skin.

Next, as for an optical equivalent scattering coefficient μs′,measurement results of a titanium oxide-dispersed polyol resin (HDI isadded) and a titanium oxide-dispersed liquid polyol (no HDI is added) ineach composition are shown in each of Tables 3 and 4. The measuredwavelength in Table 3 was 756 nm which was the wavelength used as anindex indicating the degree of oxygen saturation in blood. The measuredwavelength in Table 4 was 1,064 nm. As the polyol, a copolymer (numberaverage molecular weight: 7,000) of ethylene oxide and propylene oxideat a molar ratio of 50:50 was used. As the isocyanate compound for theresin curing, HDI was used. The addition amount of HDI was set to 3.4percent by weight to the polyol. As the filler, titanium oxide (averageparticle diameter: 0.21 μm) processed by a surface treatment usingaluminum oxide and hexamethyldisilazane was used. In the table, thedispersed amount indicates a weight ratio of the titanium oxide to thepolyol.

TABLE 3 EQUIVALENT EQUIVALENT SCATTERING SCATTERING DISPERSEDCOEFFICIENT COEFFICIENT AMOUNT OF (LIQUID) (RESIN) TiO₂ [wt %] [mm⁻¹][mm⁻¹] 0.00 0.06 0.04 0.05 0.48 0.47 0.10 0.56 0.58 0.15 0.62 0.64 0.200.70 0.72 0.25 0.78 0.76 0.50 1.28 1.25 0.75 1.37 1.38 1.00 1.54 1.51

TABLE 4 EQUIVALENT EQUIVALENT SCATTERING SCATTERING DISPERSEDCOEFFICIENT COEFFICIENT AMOUNT OF (LIQUID) (RESIN) TiO₂ [wt %] [mm⁻¹][mm⁻¹] 0.00 0.03 0.02 0.05 0.38 0.40 0.10 0.45 0.46 0.15 0.52 0.54 0.200.59 0.61 0.25 0.65 0.68 0.50 1.15 1.18 0.75 1.26 1.31 1.00 1.45 1.48

The equivalent scattering coefficient in a living body decreasesapproximately monotonically with an increase in wavelength. As for theequivalent scattering coefficient in a living body, it has been knownthat the equivalent scattering coefficient μs′ is 0.45 to 1.15 mm⁻¹ at awavelength of 756 nm, and the equivalent scattering coefficient μs′ is0.4 to 0.95 mm⁻¹ at a wavelength of 1,064 nm. As apparent from Tables 3and 4, when titanium dioxide is not dispersed in the polyol, theequivalent scattering coefficient is smaller than that of a living body,and hence the optical properties cannot be reproduced.

Therefore, the lower limit of the dispersed amount of titanium oxide tothe polyol must be an amount which at least satisfies the lower limit ormore of the equivalent scattering coefficient of a living body. That is,from Tables 3 and 4, it is found that in order to satisfy thiscondition, 0.1 percent by weight or more of titanium oxide is required.

As for the upper limit of the dispersed amount of titanium oxide to thepolyol, when the dispersed amount of titanium oxide is 0.50 percent byweight, the equivalent scattering coefficient of a living body is notreproduced. Hence, in the present invention, the dispersed amount of aninorganic oxide to the polyol is set in a range of 0.1 to 0.25 percentby weight. By controlling the dispersed amount as described above, amatching material having optical properties similar to those of a livingbody can be obtained.

In addition, as apparent from the results shown in Tables 3 and 4, sincethe resin curing has substantially no influence on optical properties,even if resin curing is not performed, optical properties similar tothose of a living body can be obtained.

Next, as for a light absorption coefficient μa, measurement results of afiller-dispersed polyol of each composition are shown in Tables 5 and 6.Table 5 shows the measurement results at a wavelength of 756 nm, andTable 6 shows the measurement results at a wavelength of 1,064 nm. Asthe polyol, a copolymer (number average molecular weight: 7,000) ofethylene oxide and propylene oxide at a molar ratio of 50:50 was used.As the isocyanate compound for resin curing, HDI was used. As thefiller, titanium oxide processed by a surface treatment using aluminumoxide and a polyol-bonded black pigment (carbon black) dispersion wereused. In the table, the dispersed amount indicates a weight ratio of theblack pigment to the polyol. The dispersed amount of titanium oxideprocessed by a surface treatment was set to 0.2 percent by weight to thepolyol. The addition amount of HDI was set to 3.4 percent by weight tothe polyol.

TABLE 5 DISPERSED ABSORPTION ABSORPTION AMOUNT OF COEFFICIENTCOEFFICIENT BLACK PIGMENT [wt %] (LIQUID) [mm⁻¹] (RESIN) [mm⁻¹] 0.00000.0007 0.0008 0.0001 0.0045 0.0042 0.0002 0.0058 0.0055 0.0004 0.00700.0068 0.0005 0.0078 0.0082 0.0010 0.0126 0.0128

TABLE 6 DISPERSED ABSORPTION ABSORPTION AMOUNT OF COEFFICIENTCOEFFICIENT BLACK PIGMENT [wt %] (LIQUID) [mm⁻¹] (RESIN) [mm⁻¹] 0.00000.0010 0.0010 0.0001 0.0045 0.0052 0.0002 0.0062 0.0068 0.0004 0.00920.0094 0.0005 0.0107 0.0119 0.0010 0.0140 0.0142

As for the light absorption coefficient in a living body (fat tissue),the absorption coefficients pa are 0.002 to 0.009 mm⁻¹ and 0.004 to0.015 mm⁻¹ at wavelengths of 756 nm and 1,064 nm, respectively. Asapparent from Tables 5 and 6, when no black pigment is dispersed in thepolyol, since the absorption coefficient is smaller than that of aliving body, the optical properties of a living body cannot bereproduced. Therefore, the optimal range of the dispersed amount of theblack pigment to the polyol is estimated as follows.

The lower limit of the dispersed amount of the black pigment to thepolyol must be an amount which at least satisfies the lower limit ormore of the absorption coefficient of a living body. That is, fromTables 5 and 6, it is found that in order to satisfy this condition,0.0001 percent by weight or more of the black pigment is required.

On the other hand, as for the upper limit, since the absorptioncoefficient of a living body is not reproduced when the dispersed filleramount to the polyol is 0.0010 percent by weight, in the presentinvention, the dispersed amount of the black pigment to the polyol ispreferably in a range of 0.0001 to 0.0005 percent by weight. From theresults shown in Tables 5 and 6, since the resin curing hassubstantially no influence on optical properties, even if resin curingis not performed, optical properties similar to those of a living bodycan be obtained. In addition, by the dispersed amount of the blackpigment within the ranges shown in Tables 5 and 6, the equivalentscattering coefficient and the acoustic properties were notsubstantially influenced.

From the above results, the optical properties of a living body can besimulated by adjustment of the dispersed amounts of titanium oxide and apigment. Hence, according to the results shown in Tables 1 to 6, thephotoacoustic matching material of the present invention includes apolyol, an inorganic oxide, and a pigment, the latter two beingdispersable in the polyol, and the dispersed amount of the inorganicoxide to the polyol is in a range of 0.1 to 0.25 percent by weight. Inaddition, the dispersed amount of the pigment to the polyol is in arange of 0.0001 to 0.0005 percent by weight.

In addition, since acoustic properties and optical properties similar tothose of a living body can be obtained, the photoacoustic matchingmaterial of the present invention is preferably prepared so that theacoustic velocity is in a range of 1,350 to 1,450 m/s, the acousticattenuation is in a range of 0.20 to 1.3 dB/cm/MHz, and with respect tolight having a wavelength in a range of 756 to 1,064 nm, the equivalentscattering coefficient and the absorption coefficient are in a range of0.4 to 1.15 mm⁻¹ and in a range of 0.002 to 0.015 mm⁻¹, respectively.

As described above, a photoacoustic matching material having lightpropagation properties and sound propagation properties similar to thoseof human tissues can be prepared by adjustment of the dispersed amountof titanium oxide. In addition, since the water absorption rate of apolyol in which titanium oxide is dispersed is low regardless of whetherthe resin curing is performed or not, a non-corrosive photoacousticmatching material can be prepared. Since the viscosity of the liquidpolyol is approximately 700 mPa·s at 25° C. regardless of whethertitanium oxide is dispersed or not, the polyol is in a gel state havingliquidity at room temperature. In addition, since the polyol and thefiller of the present invention are harmless to a human body, the liquidpolyol containing a filler dispersed therein can be used as aphotoacoustic matching material which is applied to a skin.

When a liquid-containing bag or the like is used as a matching materialto be in contact with a test portion, a bag or a container is formed,and a filler-dispersed liquid polyol is charged therein. In this case,the thickness of the bag or the container is preferably in a range ofapproximately 0.05 to 10 mm. Although a filler-dispersed cured polyol ispreferably used as a material for the bag or the container, a material,such as poly(methyl pentene), having acoustic impedance similar to thatof human tissues and transparency may also be used for a thin film.Since a sanitary aspect is also taken into consideration, at least apart which is to be in contact with a test portion is preferablyconfigured to be changeable for every subject.

Evaluation Method of Acoustic Properties

Hereinafter, an evaluation method of acoustic properties of the presentinvention will be described. As an ultrasonic transducer (transmissionsection) functioning as a probe used for acoustic property evaluation,V303 manufactured by Olympus NDT (center frequency: 1 MHz) was used. Asa hydrophone (receive section), a needle type hydrophone PAL-1384manufactured by Precision Acoustics was used. The transducer and thehydrophone were fixed in a water tank with a jig so that the centers ofsound axes thereof coincided with each other. The distance between thetransducer and the hydrophone was set to 40 mm.

When a cured polyol was measured, a cured polyol (such as a gel sheet ofa urethane resin) adjusted to have a size of 100 mm by 100 mm and athickness of 5 or 10 mm was fixed between the above experimentaltransducer and hydrophone with a jig so that an incident angle of aphotoacoustic wave with respect to the gel sheet was 0°. A sine wave(transmission voltage: 50V) of one cycle was transmitted from thetransducer using a function generator (AFG3022 manufactured byTectronix), and a received voltage value of the hydrophone in each sheetplacement was obtained using an oscilloscope (TDS3012C manufactured byTectronix). The acoustic velocity was obtained from the difference inarrival time of a received wave between gel sheet placement and no gelsheet placement using an oscilloscope. The acoustic attenuation wasobtained from the following formula.

$\begin{matrix}{{{ACOUSTIC}\mspace{14mu} {ATTENUATION}\mspace{14mu} {PER}\mspace{14mu} 1\mspace{14mu} {cm}\text{/}1\mspace{14mu} {MHz}\mspace{14mu} \left( {{dB}\text{/}{cm}\text{/}{MHz}} \right)} = {20 \times {\log \left( \frac{\begin{matrix}{{{RECEIVED}\mspace{14mu} {ACOUSTIC}\mspace{14mu} {PRESSURE}\mspace{14mu} {IN}\mspace{14mu} 10\mspace{14mu} {mm}} -} \\{{THICK}\mspace{14mu} {SHEET}\mspace{14mu} {PLACEMENT}}\end{matrix}}{\begin{matrix}{{{RECEIVED}\mspace{14mu} {ACOUSTIC}\mspace{14mu} {PRESSURE}\mspace{14mu} {IN}\mspace{14mu} 5\mspace{14mu} {mm}} -} \\{{THICK}\mspace{14mu} {SHEET}\mspace{14mu} {PLACEMENT}}\end{matrix}} \right)} \times \frac{10\mspace{14mu} ({mm})}{5\mspace{14mu} ({mm})}}} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

The acoustic properties of a non-cured liquid polyol was calculated in amanner similar to that for the cured polyol by charging a polyol in apolycarbonate cell (cell thickness: 1.5 mm) having a size of 100 mm by100 mm and an inside width of 5 or 10 mm.

Evaluation Method of Optical Properties

Next, an evaluation method of optical properties of the presentinvention will be described. A liquid polyol was charged in a quartzcell having a size of 50 mm by 50 mm and an optical path length of 5 mm,or after being charged in the quartz cell, the polyol was cured byheating at 90° C. for 1 hour, so that a cell for measurement of opticalproperties was prepared. The transmittance and the reflectance wereobtained using this cell by a spectrophotometer V-670 manufactured byJASCO Corp. In addition, the refractive index was obtained using arefractometer KPR-2000 manufactured by Shimadzu Corp. By Monte Carlosimulation, the variable setting was optimized using the results thusobtained so that the difference between the measured value and thecalculated value is minimized, and the equivalent scattering coefficientand the absorption coefficient at each wavelength were calculated.

Second Embodiment

Next, an example of use of the photoacoustic matching material of thepresent invention will be described. The form of the photoacousticmatching material may be either a liquid (including a gel at roomtemperature (approximately 25° C.)) or a solid and is not particularlylimited. That is, a liquid photoacoustic matching material containing noisocyanate compound or a solid photoacoustic matching materialcontaining an isocyanate compound is obtained. In addition, thephotoacoustic matching material is primarily used in order to fill aspace between a test portion and a probe or a probe unit of aphotoacoustic wave diagnostic apparatus. As particular examples, Example1 (FIG. 1) and Example 2 (FIG. 2) will be described.

In order to cover a test portion having a unique shape without formingany spaces therebetween, a liquid photoacoustic matching material ispreferably used. In this case, a container which houses the above liquidis also preferably formed of a solid photoacoustic matching material ofthe present invention. That is, the liquid photoacoustic matchingmaterial containing no isocyanate compound is preferably housed in thesolid photoacoustic matching material containing an isocyanate compound.The “house” in this case may indicate both cases in which the liquidphotoacoustic matching material is housed in the solid photoacousticmatching material in an air-tight state and in non-air-tight state.

For example, when a liquid-containing bag or the like is used as amatching material to be in contact with a test portion, a bag or acontainer is formed from a thin film, and a liquid titaniumoxide-dispersed polyol is charged therein. In this case, the thicknessof the thin film is preferably in a range of approximately 0.05 to 5 mm.Although a material for the thin film is not particularly limited aslong as it has acoustic impedance similar to that of human tissues,transparency, and stretchable properties, in the present invention, asolid titanium oxide-dispersed polyol is preferably used. A particularexample will be described in Example 3 (FIGS. 3A and 3B).

Since a sanitary aspect is also taken into consideration, at least apart which is to be in contact with a test portion is preferablyconfigured to be changeable for every subject.

Third Embodiment

A third embodiment of the present invention is a phantom for aphotoacoustic wave diagnostic apparatus which is a human tissuesimulation material for a photoacoustic wave diagnostic apparatuscontaining at least one photoacoustic matching material of the presentinvention. The phantom for a photoacoustic wave diagnostic apparatusincludes a cyst material (object for detection which simulates a bloodvessel or the like) and a base material. In this embodiment, the cystmaterial and the base material are formed from the same material, andthe photoacoustic matching material of the present invention ispreferably used therefor; however, the cyst material and the basematerial may be formed from different materials. According to thisembodiment, as a cyst material, a material in which a predeterminedamount of a black pigment is dispersed in the photoacoustic matchingmaterial of the present invention is used, and the absorptioncoefficient μa thereof is calculated. A particular example will bedescribed in Example 4 (FIG. 4).

As a particular example of a method for manufacturing a photoacousticmatching material of the present invention, for example, there may bementioned a method in which after titanium oxide is dispersed in apolyol received in a beaker and stirred, vacuum defoaming is performed.In addition, in phantom manufacturing, when a cyst material which is anobject for detection used as a simulated blood vessel is prepared, ablack pigment is further dispersed. When resin curing is performed, forexample, a method may be mentioned in which after vacuum defoaming isperformed, an isocyanate compound used as a curing agent is added to theabove dispersion, and a mixture thus obtained is charged in apredetermined mold and heated at 90° C. for 1 hour.

EXAMPLES

Hereinafter, although the features of the present invention will bedescribed in more detail with reference to the following examples, thepresent invention is not limited to these examples, and as long as aphotoacoustic matching material has functions and advantages similar tothose described above, materials, composition conditions, reactionconditions, and the like may be freely changed.

Example 1

FIG. 1 is a schematic view showing the case in which the photoacousticmatching material of the present invention is used for a photoacousticwave diagnostic apparatus. The photoacoustic wave diagnostic apparatusshown in FIG. 1 at least includes a test portion 1, a probe 6 which hasan acoustic wave receiving function, a light source 5, a mirror 4, alight irradiation portion 3, an electric signal processing circuit 7, asignal processing device 8, and a display device 9. A photoacousticmatching material 2 is used in contact with the test portion 1 and theprobe 6.

In the photoacoustic wave diagnostic apparatus, the test portion 1 isirradiated with light by the light irradiation portion 3 through thephotoacoustic matching material 2. When a region (cyst material) 10having high optical absorption properties is present in the test portion1, an acoustic wave 11 (photoacoustic wave) resulting from thermalexpansion by photoirradiation is generated. The probe 6 detects thisacoustic wave 11 and converts it into an electric signal (analogsignal). In addition, for example, A/D conversion of the electric signalis performed by the electric signal processing circuit 7, and imagereconstruction (generation of image data) is performed by the signalprocessing device 8. In addition, the image data is displayed using thedisplay device 9.

The photoacoustic matching material 2 of this example was formed suchthat 0.2 percent by weight of titanium oxide which was covered with afilm of aluminum oxide and which had a particle diameter of 0.21 μm and0.0005 percent by weight of a polyol-bonded black pigment (carbon black)dispersion to a polyol were dispersed therein. In addition, 3.4 percentby weight of HDI was added to the polyol, so that the preparation wasperformed. As the polyol, a copolymer (number average molecular weight:7,000) of ethylene oxide and propylene oxide at a molar ratio of 50:50was used. As the photoacoustic properties of the photoacoustic matchingmaterial in this example, the acoustic velocity was 1393.6 m/s and theacoustic attenuation was 0.57 dB/cm/MHz. In addition, at a wavelength of756 nm, the equivalence scattering coefficient μs′ and the absorptioncoefficient μa were 0.72 mm⁻¹ and 0.0082 mm⁻¹, respectively, and at awavelength of 1,064 nm, the equivalence scattering coefficient μs′ andthe absorption coefficient μa were 0.61 mm⁻¹ and 0.0119 mm⁻¹,respectively. Therefore, a photoacoustic matching material havingphotoacoustic properties approximately equal to those of human tissueswas obtained in this example.

When the photoacoustic matching material 2 is brought into contact withthe probe and a living body, the acoustic attenuation caused by an airlayer can be prevented. In addition, since the photoacoustic matchingmaterial 2 has light propagation properties and sound propagationproperties, which are similar to those of a living body, reflection atthe interface can be prevented.

Furthermore, if a filler-dispersed liquid polyol which is not cured isapplied to the surface of a living body, the generation of an air layercaused by wrinkles of a skin can be prevented when the probe ispressure-contacted therewith.

Example 2

An example in which the photoacoustic matching material of the presentinvention is bonded to a light irradiation portion and a probe of aphotoacoustic wave diagnostic apparatus will be described with referenceto FIG. 2. In FIG. 2, reference numeral 21 indicates a probe unit inwhich a light irradiation portion and an acoustic wave probe of aphotoacoustic wave diagnostic apparatus are integrated with each other.The probe unit 21 includes a light guide tube 22, an acoustic wave probe23, an acoustic matching layer 24, and a signal transmission portion 25,and a photoacoustic matching material 26 of the present invention isprovided at a front end of the probe unit 21. As the photoacousticmatching material 26, a liquid-containing bag was used which included abag formed of a thin film having a thickness of 0.05 mm andphotoacoustic matching properties and a polyol enclosed in the bag, andin addition, with respect to the polyol, 0.2 percent by weight oftitanium oxide coated with a film of aluminum oxide and 0.0005 percentby weight of a polyol-bonded black pigment dispersion were dispersed inthe polyol. As the polyol, a copolymer (number average molecular weight:7,000) of ethylene oxide and propylene oxide at a molar ratio of 50:50was used. In addition, the probe unit 21 was used in the state in whichthe photoacoustic matching material 26 was in close contact with asubject 27.

Light passing through the light guide tube 22 is radiated toward thesubject 27 and is absorbed by a light absorber, such as a blood vessel,present inside the subject, and an acoustic wave is generated by lightexcitation. This acoustic wave transmits the subject 27, thephotoacoustic matching material 26, and the acoustic wave matching layer24 and is changed into an electric signal by the acoustic wave probe 23,and this signal can be obtained through the signal transmission portion25. When the photoacoustic matching material 26 of the present inventionis not provided, and light enters the subject 27 directly from the lightguide tube 22, an acoustic wave is generated at the interface of thesubject 27. Since being superimposed on an acoustic wave generated fromthe light absorber present inside the subject, more particularly, in aregion in the vicinity of the interface of the subject, the aboveacoustic wave functions as an undesirable noise for the acoustic wavegenerated from the light absorber. As described above, when the acousticwave is generated at the interface of the subject, observation in thevicinity of the interface inside the subject cannot be easily carriedout.

By providing the photoacoustic matching material 26 of the presentinvention at the front end of the probe unit 21, the above problem canbe avoided, and furthermore, the acoustic wave can be propagated withoutcausing any problems. As the photoacoustic matching material 26, aliquid-containing bag was used which included a bag formed of astretchable thin film obtained from a cured filler-dispersed polyol anda filler-dispersed polyol enclosed in the bag. As the photoacousticproperties of the cured filler-dispersed thin film, the acousticvelocity was 1393.6 m/s, and the acoustic attenuation was 0.57dB/cm/MHz. In addition, at a wavelength of 756 nm, the equivalencescattering coefficient μs′ and the absorption coefficient μa were 0.72mm⁻¹ and 0.0082 mm⁻¹, respectively, and at a wavelength of 1,064 nm, theequivalence scattering coefficient μs′ and the absorption coefficient μawere 0.61 mm⁻¹ and 0.0119 mm⁻¹, respectively. In addition, as thephotoacoustic properties of the filler-dispersed liquid polyol, theacoustic velocity was 1442.2 m/s, and the acoustic attenuation was 0.51dB/cm/MHz. In addition, at a wavelength of 756 nm, the equivalencescattering coefficient μs′ and the absorption coefficient μa were 0.70mm⁻¹ and 0.0078 mm⁻¹, respectively, and at a wavelength of 1,064 nm, theequivalence scattering coefficient μs′ and the absorption coefficient μawere 0.59 mm⁻¹ and 0.0107 mm⁻¹, respectively. Although the lightradiated from the light guide tube 22 enters the photoacoustic matchingmaterial 26, in this case, an acoustic wave is generated at theinterface between the photoacoustic matching material 26 and the lightguide tube 22. Although light further travels toward the subject 27,since the optical properties of the subject 27 are similar to those ofthe photoacoustic matching material 26, the generation of the acousticwave at the interface therebetween is suppressed. Furthermore, since theacoustic properties of the photoacoustic matching material 26 aresimilar to those of the subject 27, the acoustic wave generated from thelight absorber inside the subject is propagated to the photoacousticmatching material 26 without being reflected and is changed into anelectric signal by the acoustic wave probe 23 through the acousticmatching layer 24, so that the signal can be obtained through the signaltransmission portion 25.

By providing the photoacoustic matching material 26 at the front end ofthe probe unit 21, the noise generated at the interface was suppressedto a level of approximately one hundredth and could be ignored. Asdescribed above, by bonding the photoacoustic matching material havinglight propagation properties and sound propagation properties similar tothose of human tissues to the probe unit, the acoustic wave generatedfrom the light absorber inside the subject is clearly obtained, and thequality of imaging inside the subject can be improved.

In this example, as the photoacoustic matching material 26, although theliquid-containing bag was used which included a bag formed of a thinfilm having acoustic matching properties and a filler-dispersed polyolenclosed in the bag, the polyol containing titanium oxide and carbonblack dispersed therein, when a cured filler-dispersed polyol similar tothat of Example 1 was used, an effect similar to that described abovecan also be obtained. Furthermore, if a filler-dispersed polyol which isnot cured is applied to the liquid-containing bag, the generation of anair layer caused by wrinkles of the liquid-containing bag can beprevented when the probe is pressure-contacted therewith.

Example 3

An example in which a container formed of the photoacoustic matchingmaterial of the present invention is applied to a photoacoustic wavediagnostic apparatus will be described with reference to FIGS. 3A and3B. A photoacoustic wave diagnostic apparatus shown in FIG. 3A includesan automatically scannable probe 38 having an acoustic wave receivingfunction, a support plate 34 of a subject used in contact therewith, alight source 37, a mirror 36, a light irradiation portion 35 whichirradiates a test portion with light, an electric signal processingcircuit 39, a signal processing device 40, and a display device 41. Inthis example, a container 32 is used which contains a liquidphotoacoustic matching material 31 therein and which has a wall surfaceformed of a resin-cured photoacoustic matching material. In thephotoacoustic wave diagnostic apparatus shown in FIG. 3A, FIG. 3B showsa cross-sectional view of the container 32 when a test portion 42 isplaced therein.

The acoustic wave diagnostic apparatus performs photoirradiation to thetest portion 42 by the light irradiation portion 35 through the supportplate 34 and the container 32. When a region 43 having high opticalabsorption properties is present in the test portion 42, an acousticwave 44 resulting from thermal expansion by photoirradiation isgenerated. The probe 38 detects this acoustic wave, and a screen displayis performed using the electric signal processing circuit 39, the signalprocessing device 40, and the display device 41.

The container 32 of this example was prepared such that 0.2 percent byweight of titanium oxide having a particle diameter of 0.21 μm andcoated with a film of aluminum oxide and 0.0005 percent by weight of apolyol-bonded black pigment (carbon black) dispersion to a polyol weredispersed therein, and 3.4 percent by weight of HDI was added to thepolyol. As the polyol, a copolymer (number average molecular weight:7,000) of ethylene oxide and propylene oxide at a molar ratio of 50:50was used. The wall thickness of the container 32 was set to 5 mm. Inaddition, in this example, a surface of the container 32 which was incontact with the test portion was at least formed of a stretchable thinfilm 33. In this example, for the thin film 33, a cured filler-dispersedpolyol similar to that for the container 32 was used. The thickness ofthe thin film 33 in this example was set to 50 μm.

As the photoacoustic properties of the photoacoustic matching materialused for the container 32 and the thin film 33 of this example, theacoustic velocity was 1393.6 m/s, and the acoustic attenuation was 0.57dB/cm/MHz. In addition, at a wavelength of 756 nm, the equivalencescattering coefficient μs′ and the absorption coefficient μa were 0.72mm⁻¹ and 0.0082 mm⁻¹, respectively, and at a wavelength of 1,064 nm, theequivalence scattering coefficient μs′ and the absorption coefficient μawere 0.61 mm⁻¹ and 0.0119 mm⁻¹, respectively. In addition, for theliquid photoacoustic matching material 31 charged in the container 32formed of the photoacoustic matching material, a polyol was used inwhich to the polymer, 0.2 percent by weight of titanium oxide coatedwith aluminum oxide and 0.0005 percent by weight of a polyol-bondedblack pigment (carbon black) dispersion were dispersed. As the polyol, acopolymer (number average molecular weight: 7,000) of ethylene oxide andpropylene oxide at a molar ratio of 50:50 was used. As the photoacousticproperties of the liquid photoacoustic matching material of thisexample, the acoustic velocity was 1442.2 m/s, and the acousticattenuation was 0.51 dB/cm/MHz. In addition, at a wavelength of 756 nm,the equivalence scattering coefficient μs′ and the absorptioncoefficient μa were 0.70 mm⁻¹ and 0.0078 mm⁻¹, respectively, and at awavelength of 1,064 nm, the equivalence scattering coefficient μs′ andthe absorption coefficient μa were 0.59 mm⁻¹ and 0.0107 mm⁻¹,respectively.

Therefore, in this example, a photoacoustic matching material havingphotoacoustic properties approximately equal to those of human tissueswas obtained.

By inserting a subject into the container 32, the noise generated at theinterface was suppressed to a level of approximately one hundredth andcould be ignored. As described above, since the container formed of thephotoacoustic matching material having light propagation properties andsound propagation properties similar to those of human tissues isbrought into contact with a test portion, the acoustic wave generatedfrom the light absorber inside the subject is clearly obtained, and thequality of imaging inside the subject can be improved.

Example 4

An example in which the photoacoustic matching material of the presentinvention is applied to a phantom for a photoacoustic wave diagnosticapparatus will be described with reference to FIG. 4. In a phantom for aphotoacoustic wave diagnostic apparatus shown in FIG. 4, cyst materials(1) to (4) designated by reference numerals 52 to 55 which were objectsfor detection used as simulated blood vessels were arranged in a basematerial 51 formed of a photoacoustic matching material having lightpropagation properties and sound propagation properties similar to thoseof human tissues. The size of the phantom was set to 120×70×50 mm. Thebase material 51 of the phantom for a photoacoustic wave diagnosticapparatus of the present invention was prepared such that 0.2 percent byweight of titanium oxide coated with a film of aluminum oxide and 0.0005percent by weight of a polyol-bonded black pigment (carbon black)dispersion to a polyol were dispersed therein, and 3.4 percent by weightof HDI was added to the polyol. As the polyol, a copolymer (numberaverage molecular weight: 7,000) of ethylene oxide and propylene oxideat a molar ratio of 50:50 was used. As the photoacoustic properties ofthe base material of this example, the acoustic velocity was 1393.6 m/s,and the acoustic attenuation was 0.57 dB/cm/MHz. In addition, at awavelength of 756 nm, the equivalence scattering coefficient μs′ and theabsorption coefficient μa were 0.72 mm⁻¹ and 0.0082 mm⁻¹, respectively,and at a wavelength of 1,064 nm, the equivalence scattering coefficientμs′ and the absorption coefficient μa were 0.61 mm⁻¹ and 0.0119 mm⁻¹,respectively. Therefore, in this example, a base material havingphotoacoustic properties approximately equal to those of human tissueswas obtained.

The cyst materials (1) to (4) were prepared by dispersing a blackpigment (carbon black) in the same material as that for the basematerial. In this case, the contrast of a detection image using aphotoacoustic wave diagnostic apparatus was evaluated by calculating theabsorption coefficient μa to the dispersed amount of the black pigment.In addition, accuracy evaluation of a photoacoustic wave diagnosticapparatus was performed by arranging the cyst materials inside thephantom. In this example, cylindrical cyst materials each adjusted tohave a diameter of 2 mm and a length of 70 mm were arranged inside thephantom. In Table 7, the absorption coefficients μa of the base materialand the cyst materials at a measurement wavelength of 1,064 nm and thecontrast ratio of the cyst material to the base material are shown.Since the absorption coefficient μa of blood is approximately 0.05 mm⁻¹,the accuracy control for hemoglobin detection of a photoacoustic wavediagnostic apparatus can be performed by using the phantom for aphotoacoustic wave diagnostic apparatus of this example. In addition, byusing the phantom for a photoacoustic wave diagnostic apparatus of thisexample, the accuracy control of a contrast ratio in a range of 5 to 20dB to the base material can be performed.

TABLE 7 ABSORPTION COEFFICIENT [mm⁻¹] CONTRAST [dB] BASE MATERIAL 0.0088— CYST MATERIAL (1) 0.0158 5.08 CYST MATERIAL (2) 0.0285 10.21 CYSTMATERIAL (3) 0.0498 15.05 CYST MATERIAL (4) 0.0890 20.10

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2010-052418, filed Mar. 9, 2010, and No. 2011-009214, filed Jan. 19,2011, which are hereby incorporated by reference herein in theirentirety.

REFERENCE SIGNS LIST

-   -   1 living body (test portion)    -   2 photoacoustic matching material    -   3 light irradiation portion    -   4 mirror    -   5 light source    -   6 probe    -   7 electric signal processing circuit    -   8 signal processing device    -   9 display device    -   10 cyst material

1. A photoacoustic matching material comprising: a polyol, an inorganicoxide, and a pigment, the latter two being dispersable in the polyol,wherein a dispersed amount of the inorganic oxide to the polyol is in arange of 0.10 to 0.25 percent by weight, and a dispersed amount of thepigment to the polyol is in a range of 0.0001 to 0.0005 percent byweight.
 2. The photoacoustic matching material according to claim 1,further comprising an isocyanate compound.
 3. The photoacoustic matchingmaterial according to claim 1, wherein the polyol is a polyether polyolwhich includes a copolymer of ethylene oxide and propylene oxide at amolar ratio in a range of 30:70 to 70:30.
 4. The photoacoustic matchingmaterial according to claim 1, wherein the inorganic oxide includestitanium oxide.
 5. The photoacoustic matching material according toclaim 1, wherein the pigment includes carbon black.
 6. The photoacousticmatching material according to claim 1, wherein the isocyanate compoundincludes hexamethylene diisocyanate.
 7. The photoacoustic matchingmaterial according to claim 1, wherein the photoacoustic matchingmaterial in the form of a liquid containing no isocyanate compound ishoused in the photoacoustic matching material in the form of a solidcontaining an isocyanate compound.
 8. A human tissue simulation materialcomprising: a polyol, an inorganic oxide, and a pigment, the latter twobeing dispersable in the polyol, wherein a dispersed amount of theinorganic oxide to the polyol is in a range of 0.10 to 0.25 percent byweight, and a dispersed amount of the pigment to the polyol is in arange of 0.0001 to 0.0005 percent by weight.